1. Field of the Invention
This invention relates to a ferrite welding rod for welding stainless steels.
2. Description of the Prior Art
Many austenite welding rods for welding stainless steels have been proposed. However, only welding rods Y410 and Y430 according to Japanese Industrial Standards, i.e., JIS Z3321, are available as standard ferrite welding rods.
Welding rod Y410 has the following composition (Hereinafter the percentage referred in this specification shall mean the weight percentage unless otherwise specified.): 0.12% or less of carbon, 0.50% or less of silicon, 0.60% or less of manganese, 0.03% or less of phosphorus, 0.03% or less of sulfur, 0.60% or less of nickel, from 11.5 to 13.5% of chromium, 0.60% or less of molybdenum, and the balance of iron.
And welding rod Y430 has the following composition: 0.10% or less of carbon, 0.50% or less of silicon, 0.60% or less of manganese, 0.03% or less of phosphorus, 0.03% or less of sulfur, 0.60% or less of nickel, form 15.5 to 17.0% of chromium, and the balance of iron.
Also, welding rod YT434NB produced by Nittetsu Yousetsu Kogyo Co., Ltd. has been known as a commercially available ferrite welding rod. Welding rod YT434NB has the following composition: 0.07% of carbon, 0.35% of silicon, 0.64% of manganese, 17.7% of chromium, 0.90% of molybdenum, 0.91% of niobium, and the balance of iron.
And especially, when welding metals of different types, for instance, when welding a ferrite stainless steel and an austenite stainless steel, welding rod Y309 according to Japanese Industrial Standards, i.e., JIS Z3321, has been used quite often. Welding rod Y309 has the following composition: 0.12% or less of carbon, 0.60% or less of silicon, from 1.00% to 2.50% of manganese, 0.03% or less of phosphorus, 0.03% or less of sulfur, from 12.0 to 14.0% of nickel, from 23.0 to 25.0% of chromium, and the balance of iron.
Weldments using the conventional welding rod, such as welding rods Y410 and Y430, used for welding ferrite stainless steels are very brittle, since carbon, oxygen and the like in a shielding gas cause the martensite precipitation when welding. There has been proposed an engineering approach, like the above mentioned welding rod YT434NB, in which niobium or titanium is added to welding rod Y430 to delay the delayed fracture, and the crystal grains are made smaller by the effect of titanium to avoid the cracking and fracture. However, the engineering approach has not produced a satisfactory result yet. In addition, weldments using these conventional welding rods exhibit the hydrogen embrittlement and reduced impact resistance when they have not undergone the heat treatment before and after welding.
The welding rod Y309 used for welding stainless steels of different types cannot be applied to a component which is subjected to a thermal cycle composed of heating and cooling processes. The deformation and cracking of the component occur, since the bulging i.e., the weld bead bulges outwardly due to the thermal cycle when an austenite stainless steel with a high thermal expansion coefficient is lapped and welded on a ferrite stainless steel with a low thermal expansion coefficient, is caused by the thermal stress or the thermal fatigue resulting from the difference in the thermal expansion coefficients. When welding rod Y410 or Y430 is used to weld the stainless steels of different types, carbon and nickel are dissolved into the weld bead from the austenite stainless steel. Thus, the impact resistance of the lap joint is reduced, since the structure in the weld bead is transformed into martensite. Further, the delayed fracture or the hydrogen embrittlement occurs when hydrogen and oxygen intrude into such martensitized weld bead.